These magazines aren't big. Use DU at 19kg/m3 you need a 0.3m3 slug at 0.1c to equal your biggest turrets total 5 second output. That's not alot of space in anyones book, not given the size of the ships.
An energy beam is far more wasteful, radiation losses, blooming and dozens more reasons. Of course as you've hand waved all that away I suppose that isn't a factor.
Here's what I think you mean about shields, your reactors generating at xKW, but the shields charge up, giving a total strength of 3 times x KW or whatever the charge up factor is. In that case surely the slug is more dangerous, all its power is concentrated in one burst, the turrets spread their power out over several seconds.
However given that your first and last paragraphs contradict each other I'm not sure what you do mean.
Re: A new Game story (From the begining)
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